I have a quiz

A box sits at the top of an unloading ramp. As the ramp is gradually tilted down from horizontal, the box will eventually start to slide down the ramp; the ramp then stops futher tilting and is held at this angle. Considering friction realistically, once the box starts to slide it is most- likely to then:

a) come to rest in a short distance due to the difference between static and kinetic.
b) continue to slide with increasing acceleration
c) Continue to slide at a constant speed because of friction, and the ramp tilt has stopped increasing.
d) Continuesto slide at a speed increasing with constant acceleration.

A box sits at the top of an unloading ramp. As the ramp is gradually tilted down from horizontal, the box will eventually start to slide down the ramp; the ramp then stops futher tilting and is held at this angle. Considering friction realistically, once the box starts to slide it is most- likely to then:

a) come to rest in a short distance due to the difference between static and kinetic.
b) continue to slide with increasing acceleration
c) Continue to slide at a constant speed because of friction, and the ramp tilt has stopped increasing.
d) Continuesto slide at a speed increasing with constant acceleration.

A box sits at the top of an unloading ramp. As the ramp is gradually tilted down from horizontal, the box will eventually start to slide down the ramp; the ramp then stops futher tilting and is held at this angle. Considering friction realistically, once the box starts to slide it is most- likely to then:

a) come to rest in a short distance due to the difference between static and kinetic.
b) continue to slide with increasing acceleration
c) Continue to slide at a constant speed because of friction, and the ramp tilt has stopped increasing.
d) Continuesto slide at a speed increasing with constant acceleration.

The answer is d).

For most combination of surfaces the coefficient of static friction is greater than the coefficient of kinetic friction. This means we will require a greater tilt of the board to move the object initially than we do to keep it moving at constant speed. (The component of the weight along the board has to be larger to overcome the maximum static friction force than it needs to be in order to keep it sliding down the board.) This means that the box will start to slide down the ramp and since the weight component of the box along the board is greater than it needs to be for constant speed (once the box has overcome the static friction), the box will accelerate down the board.